Some of the hydraulic driving systems for construction machines such as hydraulic excavators are designed to control a flow rate of a hydraulic fluid as delivered from a hydraulic pump (a main pump). Accordingly, a fluid delivery pressure of the hydraulic pump becomes higher by a target differential pressure than a load pressure of an actuator to which the highest load pressure is to be assigned among a plurality of actuators. Such flow rate control is called load-sensing control. The hydraulic driving systems in which the load-sensing control is performed are adapted to maintain a predetermined differential pressure across each of a plurality of flow control valves via a pressure compensating valve disposed for the flow control valve independently. During combined operations control for simultaneously driving the actuators, the hydraulic driving systems can thus supply the hydraulic fluid to the actuators at a ratio commensurate with an opening area of each flow control valve, irrespective of a magnitude of the actuator load pressures.
Patent Document 1, for example, describes such a hydraulic driving system adapted to perform the load-sensing control. The hydraulic driving system described in Patent Document 1 is configured so that a differential pressure (hereinafter referred to as the load-sensing differential pressure) between a fluid delivery pressure of a hydraulic pump and a load pressure of an actuator to which the highest load pressure is to be assigned among a plurality of actuators is guided as a target compensation differential pressure to pressure-receiving portions constructed so as to operate pressure compensating valves in a direction to increase in opening area. The hydraulic driving system is also configured so that the target compensation differential pressure across each of the pressure compensating valves is set to be the same value equivalent to the load-sensing differential pressure. Thus, a differential pressure across each of a plurality of flow control valves is held at the load-sensing differential pressure level. During combined operations control for simultaneously driving the actuators, therefore, even if the fluid delivery pressure of the hydraulic pump is insufficient (this state is hereinafter referred to as saturation), a decrease in load-sensing differential pressure according to a particular degree of the saturation uniformly reduces the target compensation differential pressures of the pressure compensating valves (i.e., the differential pressures across the flow control valves), thus enabling a delivery rate of the hydraulic fluid from the hydraulic pump to be redistributed to a ratio of the flow rates demanded from the actuators.
In addition, the pressure compensating valves of the hydraulic driving systems in which the load-sensing control is performed are usually configured so that as described in Patent Document 1, the valve will fully close when a spool operates in a direction to reduce an opening area of the valve and reaches a stroke end of the spool.
In contrast to the above, Patent Document 2 describes a hydraulic driving system configured so as not to fully close a pressure compensating valve even after a spool has operated in a direction to reduce an opening area of the valve and reached a stroke end of the spool.